How Things Work: Slot Machines

Cherry… cherry… lemon! Slot machines, or “one-armed bandits,” have long been a part of America’s gambling culture. While most casino games require skill, large bets, and the ability to think under pressure, slot machines are engaging because of their comparative simplicity.

For decades, the players of slot machines were mostly the wives and girlfriends of serious gamblers. Though they date back to the turn of the 20th century, it took until the ’90s for slots to really take off.

“One thing that really broke through all the clutter was when they came up with video machines,” said Duncan Brown, a game designer at Leading Edge Design.

Mechanical and video slot machines work differently on the inside, but both provide the same fundamental playing experience. A gambler inserts money and then pulls the machine’s handle, thus spinning its wheels. The wheels stop one at a time and the machine dispenses money based on the symbols of the payline.

Different symbol combinations amount to different payouts.

Traditional machines

The central component of any older slot machine is a metal shaft. The metal shaft holds the three wheels that a player sees on the face of the machine, where the symbols are located. Each wheel is connected to a notched disc, which is basically a large gear around the shaft.

The mechanism that spins the wheels, called the kicker, looks like a rectangular plate. When the machine is at rest, one end of the kicker is stuck inside a notch in each of the three discs.

When a player inserts money into the machine, a coin detector frees the handle. As soon as a player pulls, the kicker rotates a piece called the control cam. The control cam releases the cam plate, which holds the three stoppers out of the way of the wheels.

The kicker tilts slightly forward, momentarily turning the three discs towards the front of the machine. It then jerks backward and sets them each spinning in the opposite direction.

The motion of the kicker is on par with a golf club hitting a ball, if the club had a very short backswing and a comparatively larger foreswing.

All the while, a spring is pulling the control cam slowly back to where it started. When it gets there, the control cam resets the cam plate so that the three brakes are no longer restrained. The breaks stop their respective wheels one at a time instead of all at once because each was initially connected to the cam plate on a different catch.

Each of the notched discs has one especially deep notch corresponding to the jackpot. However, the machine is built so that all three stoppers must land on their respective jackpot notches to activate the jackpot.

If the second stopper lands on the jackpot, a catch in the first stopper prevents the second from fully sinking into the notch, unless the first stopper has also landed on the jackpot.
Similarly, the third stopper will only sink into the deep notch if the first and the second are already in their respective notches.

Coins accumulate in a transparent case supported by the third stopper. When the third stopper descends into the jackpot notch, it slips out of the way of the case and releases the jackpot.
Usually each disc has 22 notches, so the chance of hitting a jackpot is one in 10,648 (223). Most machines offer smaller payouts in addition to the jackpot. This requires a more complicated version of the basic design.

Modern slots

In new slot machines, the handle serves no mechanical purpose; it might as well be a button.
“It’s all just a bunch of random numbers, basically,” Brown said. “It’s all just math.”

Even before a player places a bet, an internal random number generator is actively churning out numbers on the scale of several billion, each in less than a hundredth of a second. As soon as you pull the handle, the computer takes note of its next three random numbers.

The computer divides each number by the same value, typically 64. The remainder of each quotient is what determines where its wheel will stop.

Though there are 64 possibilities, the player still only sees 22 spaces on each wheel.

Multiple remainders are linked to each blank (losing) stop, but only one of the 64 is linked to the jackpot. The odds of hitting the jackpot are one in 262,144 (643).

For this reason, video slot machines are said to be weighted, meaning that the odds are worse than they seem.

Though hitting the jackpot is extremely unlikely, it’s easy for slot players to win small payouts. The payback percent of a slot machine is the portion of your money you would win back if you played for an infinite amount of time. State laws regulate slot machines by requiring a minimum payback percentage, typically within the 80s.

New slot machines provide a lot of options for each spin. Video machines allow the player to choose different payout patterns instead of the traditional three-in-a-row.

The machines with the largest jackpots are linked within statewide networks. Players can choose the size of their bet, the amount of wheels, and the amount of paylines.

Many of these machines’ payout amounts increase as more people play without hitting the jackpot.

Most slot machines are set to only reward the jackpot under the maximum bet available. “You can make much longer odds,” Brown said. “A lot of people don’t realize that.”

It’s a myth that slot machines can be due to hit the jackpot. The odds are the same for every spin, even after a string of losses. It’s similar to flipping a coin; 100 heads in a row don’t increase the odds of getting tails.

But even if the odds are unwavering and the handles are just for show, slot machines are extremely popular. They’ve got a hold on the American gambler, and not even math can explain that.